Three of the warmest years of the 20th century were bunched in the 1990s. Does this reflect a long-term warming of the globe by carbon dioxide and other greenhouse gases, as many atmospheric scientists have contended? Or was the hot spell just a random, unexceptional fluctuation in the weather? A study published last week in Nature magazine by climatologist Michael Mann and colleagues from the University of Massachusetts, Amherst, may help melt away any lingering doubt about global warming. The scientists developed what amounts to a time-traveling thermometer. Applying innovative statistical tools to reams of evidence gathered from ancient ice samples,...